X-ray radiation has been widely used in imaging applications such as medical diagnosis, security screening, and industrial inspection. Current x-ray imaging systems typically consist of an x-ray source, an object stage, and a digital detector/film. The spatial resolution of current imaging systems is limited by the size of x-ray focal spot characteristics, detector pixel pitch, and imaging geometries. It is desirable to improve the spatial resolution of current x-ray imaging systems. By reducing the focal spot size in an x-ray imaging system, the spatial resolution of the imaging system can be increased.
High-resolution x-ray micro computed tomography (micro-CT) is now routinely used for in vivo imaging in preclinical cancer studies of small animals with high spatial and contrast resolution. Its capability for in vivo imaging of lung and colon cancers in mouse models has recently been demonstrated. By using contrast medium, micro-CT is effective in revealing soft tissues.
A typical micro-CT scanner comprises a microfocus x-ray source, a sample stage, and a flat panel x-ray detector. The resolution of the scanner is determined by parameters including the x-ray focal spot size (i.e., the size of the anode area that emits x-ray radiation), the geometry, and the detector resolution. Although x-ray sources with an effective focal spot size of less than 10 μm are now commercially available, in practice the imaging resolution is constrained by motion-induced blur in live objects and by concerns of the total x-ray dose, especially for longitudinal studies. For ungated micro-CT imaging of live mice, prior experiments have shown that the imaging artifacts due to respiratory and cardiac motions may completely obscure the anatomical details within the region of the lung and heart. Motion-induced artifacts can be reduced by gating the x-ray exposure in synchronization with physiological signals. A recent study using a respiratory and cardiac gated micro-CT with a conventional thermionic x-ray source reported spatial resolution of ˜100 μm. Further increasing the resolution is partially limited by the temporal resolution and available flux of the x-ray source.
Carbon nanotubes (CNTs) possess extraordinary physical and chemical properties. They have been demonstrated as excellent electron field emitters due to their high geometric aspect ratio, high mechanical strength, and chemical stability. They have been employed as efficient electron field emission cathodes in the development of x-ray sources. Diagnostic quality x-ray radiation with temporal resolution up to a microsecond has been successfully demonstrated.
Carbon nanotube based field emission x-ray sources have been shown to have several intrinsic advantages over the current x-ray tubes with thermionic cathodes. These include high temporal resolution and capabilities for spatial and temporal modulation. In addition, the ease of electronic control of the radiation readily enables synchronized and/or gated imaging which is attractive for imaging of live objects. However, known experiments have demonstrated a deficiency in achieving fine or small focal spots in x-ray sources. Therefore, it is desirable to provide field emission x-ray sources having very fine or small focal spots. Such field emission x-ray sources can provide improved resolution for obtaining more detailed images of objects, particularly small objects.